Semiconductor lasers that emit light in the long wavelength visible (red) and near infrared spectra have been known for many years. To date, however, it has been difficult to produce such lasers that emit light in the short wavelength, blue region of the visible spectra. A wide variety of applications await such "blue" lasers, should they become commercially available.
Certain II-VI semiconductor compounds such as zinc selenide sulphide (ZnSeS) have been considered promising candidates for blue lasers. In an article entitled "Blue-green laser diodes", Applied Physics Letters., v. 59, Sep. 9, 1991, M. A. Haase et al. describe achieving lasing action at a wavelength of 490 nm with a ZnSe based structure under pulsed current injection at 77 K. More recent advances in this material system using a remote plasma source for the introduction of radical nitrogen (N) atoms have resulted in the demonstration of pulsed laser operation at room temperature at a wavelength of 480 nm.
The ZnSeS based structures have several limitations, however, due to conduction band alignment and the activation energy of defects. The confinement energy of the cladding layers limits efficient operation of the ZnSeS system to a wavelength of 530 nm, as the electron confinement is too small for efficient operation of laser diodes at 480 nm or shorter wavelengths. The low activation energy of defects in the crystal lattice of such a material limits the processing and growth temperature of the structure to no more than a few hundred degrees Celsius.
The ZnSeS materials are further complicated by the problems associated with growth on a gallium arsenide (GaAs) substrate. The interface between the ZnSeS and GaAs results in a cross doping of the compounds. The Ga and As act as dopants in the ZnSeS and the Zn, Se and S act as dopants in the GaAs. As a result the interface between the two compounds becomes quite resistive. Laser diodes fabricated with this interface require operating voltages in excess of 20 V. The heat dissipated from this is also high, preventing continuous wave (CW) operation at room temperature.
An alternative to the II-VI compounds for blue light emission are the III-V semiconductor compounds, such as AlGaInN. The advantages of the III-V material systems are several. First, they include materials having large direct bandgaps ranging from 2 eV to 6 eV. Second, large energy differences between the valleys of the lowest direct bandgaps and those of the lowest indirect bandgaps exist. Third, electrons and holes can be satisfactorily confined in heterostructures, including quantum well structures, due to large confinement energies. Fourth, production of these materials is compatible with metal organic chemical vapor deposition (MOCVD) growth reactors. Fifth, the materials typically have low resistance to n-type and p-type doping. Finally, the III-V materials can avoid the cross doping and defect creation and propagation problems that plague the II-VI materials.
AlGaInN materials have so far been grown only on mismatched substrates such as Si, SiC, single crystal Al.sub.2 O.sub.3 and MgO. Mismatched crystal lattices tend to have defects that absorb light, lowering the efficiency of light generation and reducing lasing potential. Defects also tend to propagate through crystals, lowering the useful life of the crystals.
Another difficulty is that AlGaInN type materials typically have a native wurtzite, or generally hexagonal, crystal lattice, which has few convenient cleavage planes to form mirrored facets for Fabry-Perot reflection and which cannot be easily grown to exactly layered depths for quantum wells or other cladding confined structures. For laser diodes, it is desirable that the heterostructure material have a generally cubic zinc blende crystal lattice. While in AlGaInN materials the cubic structure is metastable at typical production temperatures, and thus not impossible to form in principle, to actually produce such a cubic crystal lattice material by epitaxial growth techniques appears to require a substrate that is not only lattice matched to the AlGaInN type material but also has a cubic lattice that induces the growth of a cubic substrate lattice of the AlGaInN type material layers. Substrates which have commonly been used for group III-V high bandgap semiconductors, such as basal plane {0001} sapphire (Al.sub.2 O.sub.3) and .alpha.-SiC, have the disadvantage of producing the less desirable wurtzite crystal lattice structure.
In U.S. Pat. No. 5,146,465, Kahn et al. describe growing layers of AlGaN with alternating concentrations of Al and Ga on an AlN buffer layer which, in turn, was grown on an Al.sub.2 O.sub.3 substrate. Kahn et al. appear to overcome some of the difficulties inherent in the wurtzite lattice by polishing walls of the device and adding mirrors. However, cleaved facets, when available, are inherently better mirror surfaces than etched or polished surfaces because of their lower defect density. In U.S. Pat. Nos. 5,173,751 and 5,274,251, Ota et al. take advantage of the lattice constant of .alpha.-ZnO, which falls between that of several III-V nitride materials, allowing mixing of those materials in proportions calculated to match the substrate lattice of ZnO, to form AlGaInN or AlGaNP layers. The resultant crystal lattice appears to be wurtzite in form.
In U.S. Pat No. 4,862,471, Pankove describes the growth on a gallium phosphide (GaP) substrate of gallium nitride (GaN). Indium nitride (InN) or aluminum nitride (AlN) layers are similarly grown to form a quantum well light emitting device. Similarly, U.S. Pat. No. 5,076,860 to Ohba et al. teaches a compound semiconductor material of GaAlBNP with a zincblende (cubic) crystal lattice, grown on a GaP substrate. Ohba et al. describe several different materials, including a GaAlN semiconductor grown on a BP substrate and the growth of group III-V materials having ordered bonds but non-lattice matched crystals. U.S. Pat. No. 5,042,043 to Hatano et al. describes a semiconductor laser formed from alternately stacking BP and GaAlN layers to form Ga.sub.x Al.sub.y B.sub.1-x-y N.sub.z P.sub.1-z material on a GaP substrate. In all of these cases, the use of GaP as a substrate results in a severe lattice mismatch with the nitride material layers (about 20% mismatch for GaN). In an article entitled "High-Efficiency Aluminum Indium Gallium Phosphide Light-Emitting Diodes", Hewlett-Packard Journal, August 1993, pp. 6-14, R. M. Fletcher et al. describe gallium arsenide phosphide doped with nitrogen (GaAsP:N) and gallium phosphide doped with nitrogen (GaP:N).
In an article entitled "Luminescence quenching and the formation of the GaP.sub.1-x N.sub.x alloy in GaP with increasing nitrogen content", Applied Physics Letters, Vol. 60, No. 20, May 18, 1992, pp. 2540-2542, J. N. Baillargeon et al. teach N doping in GaP of up to 7.6% using molecular beam epitaxy (MBE), and note that increasing nitrogen content tends to shift emission spectra lower above a certain nitrogen concentration. Similarly, X. Liu et al. describe observing a red shift in emission spectra from GaP:N as nitrogen (N) concentration is increased in "Band gap Bowing in GaP.sub.1-x N.sub.x alloys", Applied Physics Letters, Vol. 63, No. 2, Jul. 12, 1993, pp. 206-210. Difficulties are encountered when mixed compounds having significant amounts of both nitrogen and other group V elements are attempted. Miyoshi et al. describe a miscibility gap for growth of GaP.sub.1-x N.sub.x for x.gtoreq.0.04 in an article entitled "Metalorganic vapor phase epitaxy of GaP.sub.1-x N.sub.x alloys on GaP", Applied Physics Letters, Vol. 63, No. 25, Dec. 20, 1993, pp. 3506-3508.
Of primary concern to reliable visible laser diode operation is the optimization of the substrate and growth buffer layers. To form a semiconductor material having desirable lasing properties, it is advantageous to grow such a material on a lattice matched substrate in order to avoid the promulgation of defects that absorb light. It is also desirable, whenever possible, that the substrate promote growth in the semiconductor material of a cubic crystal lattice that facilitates the relatively easy formation of cleaved reflective facets for defining resonant laser cavities.